Semiconductor wafer having a thin die and tethers and methods of making the same

ABSTRACT

A semiconductor wafer ( 70 ) that includes a support body ( 72 ), at least one thin die ( 20, 60 ), and a plurality of tethers ( 78, 178 ). The support body ( 72 ) is made of a semiconductor material. The thin die ( 20, 60 ) has a circuit ( 21 ) formed thereon and has an outer perimeter ( 74 ) defined by an open trench ( 76 ). The open trench ( 76 ) separates the thin die ( 20, 60 ) from the support body ( 72 ). The tethers ( 78, 178 ) extend across the open trench ( 76 ) and between the support body ( 72 ) and the thin die ( 20, 60 ). A method of making a thin die ( 20, 60 ) on a wafer ( 70 ) where the wafer ( 70 ) has a support body ( 72 ), a topside ( 82 ) and a backside ( 90 ). A circuit ( 21 ) is formed on the topside ( 82 ) of the wafer ( 70 ). The method may include the steps of: forming a cavity ( 88 ) on the backside ( 90 ) of the wafer ( 70 ) beneath the circuit ( 21 ) that defines a first layer ( 92 ) that includes the circuit ( 21 ); forming a trench ( 76 ) around the circuit ( 21 ) on the topside ( 82 ) of the wafer ( 70 ) that defines an outer perimeter ( 74 ) of the thin die ( 20, 60 ); forming a plurality of tethers ( 78, 178 ) that extend across the trench ( 76 ) and between the wafer support body ( 72 ) and the thin die ( 20, 60 ); and removing a portion of the first layer ( 92 ) to define the bottom surface ( 75 ) of the thin die ( 20, 60 ).

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is related to the following co-pending andcommonly assigned patent application, which is hereby incorporated byreference herein: application Ser. No. ______, entitled “METHOD OFSEPARATING AND HANDLING A THIN SEMICONDUCTOR DIE ON A WAFER,” filed onsame date herewith, by Shiuh-Hui Steven Chen, Cheryl Field, Didier R.Lefebvre, and Joe Pin Wang, attorney's docket number AP01985.

FIELD OF THE INVENTION

[0002] This invention in general relates to the making and handling of avery thin semiconductor die and, more particularly, to an improveddevice and procedure for fabricating, separating and transporting verythin dice for better throughput and yield.

BACKGROUND OF THE INVENTION

[0003] As technology progresses, integrated circuits are being formed onsmaller and thinner semiconductor dice for a variety of applications.Relatively thin integrated circuits (ICs) or semiconductor dice, alsoknown as “ultra-thin” or “super-thin” ICs or dice, are used inapplications such as smart cards, smart labels, sensors, and actuators.A thin die for sensors is described in pending application Ser. No.09/629,270, filed on Jul. 31, 2000, entitled “Strain Gauge” by Shiuh-HuiSteven Chen, et al., incorporated herein by reference in its entirety.There, a relatively thin semiconductor die with piezo-resistors act tomeasure the pressure of fluids in vehicles. The thin semiconductor dieis bonded to a stainless steel port in order to measure diaphragmdeformation.

[0004] For smart card applications, the thickness of the die may be aslow as 100 micrometers (μm). In the future, it is anticipated that aneven smaller thickness will be necessary. For sensors, a thin die mayhave a thickness of between 5 and 50 μm as described in application Ser.No. 09/629,270.

[0005] When making and handling a very thin semiconductor die, care mustbe taken not to fracture or otherwise damage the die. Currently, a needexists for improved methods and procedures to fabricate, separate, andtransport a thin die for high volume applications where automatedtechniques are required to produce high throughput and acceptableyields.

[0006] It is known to separate and handle integrated circuits on thinsemiconductor die by mechanical grinding, chemical etching and dryetching with the assistance of adhesive or UV related release tapes andcarrier wafers. Some of the approaches taken in the electronics industryto separate thin wafers into dice and handle thin dice include dicing bycutting and dicing by thinning. In dicing by cutting, a dicing tape ismounted on frames. The wafers are mounted to the dicing tape, backsidedown. Dicing is carried out by sawing, laser cutting, dry etch, etc.After cutting, the dice are separated on the dicing tape and sent to theassembly line on a wafer frame for pick and place. The thin die is thenejected from the backside of the tape with the help of an ejector pinand picked by a vacuum tip. An example of this process flow is describedin Muller et al., “Smart Card Assembly Requires Advanced Pre-AssemblyMethods,” SEMICONDUCTOR INTERNATIONAL (July 2000) 191.

[0007] In dicing by thinning, trenches are etched or sawed on thetopside of a device wafer. Laminating tapes are then placed on a carrierwafer for mounting the carrier wafer to the topside of the device wafer.The bottom side of the device wafer is then thinned until the topsidetrenches are opened from the bottom side. A second carrier wafer is thenmounted to the bottom side of the device wafer by a high-temperaturerelease tape. The first carrier wafer is removed and then the thin dicecan be removed by locally heating a vacuum-picking tool. An example ofthis process flow requiring multiple carrier wafers and tape transfersis described in C. Landesberger et al., “New Process Scheme for WaferThinning and Stress-Free Separation of Ultra Thin ICs,” published atMICROSYSTEMS TECHNOLOGIES, MESAGO, Dusseldorf, Germany (2001).

[0008] Alternatively, it has been known to saw or cut a carrier waferinto carrier chips, each of them carrying a thin die. In this case, thecarrier chip is removed after die bonding by thermal release of theadhesive tape. An example of this process flow is described in Pinel etal., “Mechanical Lapping, Handling and Transfer of Ultra-Thin Wafers,”JOURNAL OF MICROMECHANICS AND MICROENGINEERING, Vol. 8, No. 4 (1998)338.

[0009] Conventional procedures have been met with a varying degree ofsuccess. The combination of carrier transfers and tape transfersnecessitate multiple steps with long cycle times and yield loss.Moreover, the use of heat release and other tapes may exhibitunacceptable residual adhesion. Further, when used in combination withan ejector pin, the edges may not delaminate from the tape due to thelack of flexural rigidity of the thin die and due to the die's smallsize in the in-plane directions. The small size of the die may alsolimit the net suction force that could be exerted by the vacuum tip toovercome residual tape adhesion. With regard to conventional dicing andwafer sawing methods, these steps often result in damage to the thin diethat causes device failure or performance degradation. Conventionalejector pins may exert excessive stress that damages the thin die, alsocausing cracking and device failure. Carrier transfer or tape transfermay lead to die contamination on both sides of the die. Multipletransfers by wafer carriers typically lead to lower yield due toincreased handling and contamination. In the case of a very thin die forsensor applications, organic adhesive may leave residue on the diesurface, causing poor bonding with the surface being measured.

[0010] It is, therefore, desirable to provide an improved device andmethod of fabricating, separating and handling very thin dice toovercome most, if not all, of the preceding problems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is an enlarged view of one embodiment of a thinsemiconductor-sensing die with an array of strain gauges positioned in aWheatstone bridge arrangement;

[0012]FIG. 2 is an enlarged view of another embodiment of a thinsemiconductor-sensing die with a single transverse strain gauge.

[0013]FIG. 3 is a side view of a thin semiconductor-sensing die mountedon a diaphragm.

[0014]FIG. 4 is an exploded partial top view of one embodiment of awafer having a thin die with tethers.

[0015]FIG. 5 is an exploded partial top view of another embodiment of awafer having a thin die with tethers.

[0016] FIGS. 6A-6D are cross-sectional views of a process to formtethers that extend between a support body and a thin die of a wafer.

[0017]FIG. 7 is a top view of one embodiment of a rigid backing for awafer of the present invention.

[0018]FIG. 8 is a cross-sectional view of one embodiment of a diehandler for pick and place operations.

[0019] FIGS. 9A-9D are side views of one procedure of the presentinvention for separating and extracting a thin die from a wafer.

[0020] FIGS. 10A-10D are side views of one procedure of the presentinvention for transporting and installing a thin die on a surface.

[0021] While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0022] What is described is an improved device and method of making andhandling a thin semiconductor die including the fabrication, separationand transfer of such die. For purposes of illustration and description,an example of an application of a thin semiconductor die is describedbelow in the context of a thin semiconductor-sensing die for sensing thepressure of fluids in a vehicle. However, the present invention is notlimited to the making and handling of dice for sensors but may alsoapply to other thin dice applications such as smart cards, smart labels,actuators, and multi-thin wafer designs. One of ordinary skill in theart having the benefit of this disclosure will realize that the devicesand procedures described herein for the making and handling of thin dicecould be used in other semiconductor applications.

[0023] To this end, in one embodiment there is a semiconductor waferthat includes a support body, at least one thin die, and a plurality oftethers. The support body is made of a semiconductor material. The thindie has a circuit formed thereon and has an outer perimeter defined byan open trench. The open trench separates the thin die from the supportbody. The tethers extend across the open trench and between the supportbody and the thin die.

[0024] The support body may have a first thickness and the thin die mayhave a second thickness, wherein the second thickness is substantiallyless than the first thickness. In one embodiment, the tethers may besubstantially triangular in shape but other shapes may be used such assubstantially rectangular, elliptical, semi-circular, or square. It ispreferred that the portion of the tether that extends across the opentrench has its smallest width adjacent to the outer perimeter of thethin die. This provides a cohesive failure point (or break point) of thetether to be along the outer perimeter of the thin die to prevent anyresidual overhangs during subsequent pick and place operations.Moreover, this cohesive failure point should mean that the tether itselfbreaks rather than being peeled from the thin die. The tether may alsobe patterned such that at least a portion of a groove or hole extendsinto the portion of the tether that goes across the open trench. Thetethers may be made of a material such as a polyimide although othermaterials may be used to support the thin die to the support body.

[0025] In another embodiment, there is a method of making a thin die ona wafer where the wafer has a support body, a topside and a backside. Acircuit is formed on the topside of the wafer. The method may includethe steps of: forming a cavity on the backside of the wafer beneath thecircuit that defines a first layer that includes the circuit; forming atrench around the circuit on the topside of the wafer that defines anouter perimeter of the thin die; forming a plurality of tethers thatextend across the trench and between the wafer support body and the thindie; and removing a portion of the first layer to define the bottomsurface of the thin die.

[0026] In another embodiment, there is a method of forming tethers on awafer to retain a thin die to a support body of the wafer. The wafer hasa topside and a backside. The thin die is positioned adjacent to thetopside of the wafer. The method may include the steps of: forming acavity on the backside of the wafer beneath the thin die that defines afirst layer that includes the thin die; forming a trench around the thindie on the topside of the wafer that defines an outer perimeter of thethin die and extends between the thin die and the support body;patterning a polyimide material on the top surface of the wafer todefine the tethers, the tethers extending across the trench and betweenthe thin die and the support body; and removing a portion of the firstlayer to expose the trench such that the tethers provide the attachmentbetween the thin die and the support body.

[0027] Now, turning to the drawings, an example use of thinsemiconductor dice will be explained and then a wafer with a thin dieand tethers along with a method of separating and handling the thin diewill be explained.

[0028] Example Use of Thin Semiconductor Dice

[0029] For purposes of illustration and description, a thinsemiconductor die will be explained in the context of sensors formeasuring the pressure of fluids in a vehicle. Such a thin die forsensors is described in detail in pending application Ser. No.09/629,270, filed on Jul. 31, 2000, entitled “Strain Gauge” by Shiuh-HuiSteven Chen, et al., incorporated herein by reference in its entirety.

[0030] An example of a thin semiconductor die is shown in FIG. 1. Thethin semiconductor die 20 in this example is a die for a sensor thatmeasures the pressure of fluids in vehicles and may range from 5 to 50micrometers (μm) thick. The die 20 has sufficient structural strengthand integrity to support one or more strain gauges 22, 24, 26, 28. Inthis case, the die 20 is generally square and has a geometric center 30.Metal bond pads 32, 34, 36, 38 are positioned in proximity and adjacentto the corners of the die 20. A set, series, or array of silicon oxideopenings providing electrical contacts 42, 44, 46, 48 are disposed andsecurely positioned underneath the pads 32, 34, 36, 38. The die 20 hassemiconductors 52, 54, 56, 58 (such as P+ doped silicon-containinginterconnects) that provide interconnects between the strain gauges 22,24, 26, 28 and the electrical contacts 42, 44, 46, 48.

[0031] The die 20 illustrated in FIG. 1 has strain gauges 22, 24, 26, 28with interconnected resistors positioned in a Wheatstone bridgearrangement. The gauges 22, 24, 26, 28 measure strain in response to andinduced by pressure of a fluid, such as fluid in a vehicle. Accordingly,referring to FIG. 3, the thin semiconductor-sensing die 20 may bemounted to a fluid responsive diaphragm 40. The thinsemiconductor-sensing die 20 and fluid responsive diaphragm 40, and howit may interconnect with a fluid housing, is further described inapplication Ser. No. 09/629,270, filed on Jul. 31, 2000, entitled“Strain Gauge” by Shiuh-Hui Steven Chen, et al. In sum, the fluidresponsive diaphragm 40 can be positioned to contact the sensed fluid inthe vehicle. These fluid responsive diaphragms are preferably made of acorrosion-resistant material (such as stainless steel) that will notreadily corrode in the fluid being sensed.

[0032] A symmetrical pressure-conductive coupling 50 can be provided toconnect the semiconductor die to the diaphragm. The coupling 50 mayinclude a corrosive-resistant pressure-conductive electricallyinsulating material to conduct and transmit the sensed pressure from thediaphragm to the thin semiconductor-sensing die 20. A suitable coupling50 is made of fused glass frit and screen-printed on the diaphragm 40.Glass frit is useful because it electrically isolates and preventsshorts from the metal diaphragm 40.

[0033] Another embodiment of a thin semiconductor-sensing die 60 isshown in FIG. 2. The thin semiconductor-sensing die 60 as shown in FIG.2 is structurally and functionally similar to the one shown in FIG. 1but has a single transverse strain gauge 62. The single transversestrain gauge 62 is registered and positioned in alignment with thegeometrical center 64 of the die 60. This helps minimize electricaleffects of thermal stress on the transverse strain gauge duringmeasuring and operation of the vehicle. Here, the transverse straingauge can include a single four contact resistor element oriented tomaximize response to pressure induced stresses through shear stresseffects. A further description of the thin semiconductor sensing die 60and strain gauge 62 are provided in application Ser. No. 09/629,270,filed on Jul. 31, 2000, entitled “Strain Gauge” by Shiuh-Hui StevenChen, et al., incorporated herein by reference in its entirety.

[0034] As with other thin semiconductor dice, there is a continuing needto improve the separation and handling of a thin die after fabricatingthe integrated circuit thereon. In particular, there is an ongoing needto increase throughput in a low cost automated environment and toprovide better yields in such an environment.

[0035] Wafer with Thin Die and Tethers

[0036] A new device and process has been developed to assist inseparating a thin semiconductor die from a wafer. An integrated circuit21 is initially formed on a standard wafer. Further fabricationprocesses to help in subsequent separation of a die (that includes thecircuit) from the wafer are illustrated in the top views of FIGS. 4 and5 and in the cross-sectional views of FIGS. 6A-6D. The device andprocess includes the formation of thin tethers around the perimeter ofthe die. This allows for easier separation of the die from the wafer insubsequent processes. Again, for purposes of illustration, thedescription and figures are shown in the context of the thinsemiconductor die 20 described above in FIG. 1. One of ordinary skill inthe art with the benefit of this disclosure will recognize, however,that the present invention applies to other thin die applications.

[0037] Referring to FIG. 4, an exploded portion of a semiconductor wafer70 is shown having a support body 72 made of a semiconductor materialand at least one thin semiconductor die 20. The thin die 20 has anintegrated circuit (generally referenced as 21) formed thereon. The thindie 20 also has an outer perimeter 74 defined by an open trench 76. Theopen trench 76 separates the thin die 20 from the support body 72 of thewafer 70. A plurality of support tethers 78 extend across the opentrench 76 and between the support body 72 and the thin die 20. In oneembodiment, as will be seen in the cross-sectional views of FIGS. 6A-6D,the thickness of the thin die 20 is substantially less than thethickness of the support body 72.

[0038] The tethers 78 may have a variety of geometric patterns andsizes. In one embodiment, as shown in FIG. 4, the tethers 78 may besubstantially triangular. Here, the substantially triangular tethers 78have a base 80 that is formed on the topside 82 of the wafer 70 and atip 82 that extends across the open trench 76 and onto the die 20. Thetip 82 of the tether 78 may be patterned so that it is partially cutoffto limit the portion of the tether 78 extending on the die 20. Thetether 78 should, however, extend sufficiently onto the die 20 to allowthe die 20 to be retained to the wafer support body 72. This attachmentshould be sufficient to withstand normal shipping and handlingrequirements for a standard wafer. In one embodiment, for a die 20having a thickness of about 15 μm, each of the tethers extend at least10 μm over the outer perimeter 74 of the die 20.

[0039] In another embodiment, as shown in FIG. 5, a tether 178 is alsosubstantially triangular but is patterned with grooves 184. Thesubstantially triangular tethers 178 have a base 180 that is formed onthe topside 82 of the wafer 70 and a tip 182 that extends across theopen trench 76 and onto the die 20. The grooves 184 are at leastpartially formed in the portion of the tether 178 that extends over thetrench 76. The grooves 184 define a neck 186 that extends between thetwo grooves 184. The benefit of including grooves 84 in the formation ofthe tethers 178 is that they allow for better separation of the die 20from the wafer 70 during pick and place operations. Although thespecific width of the neck 186 is application specific, in oneembodiment for a thin die 20 having a thickness of about 15 μm, thewidth of the neck 186 may have ranges between 10 and 40 μm. What isimportant is that a cohesive failure point (or break point) of thetethers 178 be along the edge of the semiconductor die and such that thetether itself breaks rather than being peeled from the thin die duringpick and place operations. This break point should be sufficiently wideto withstand normal shipping and handling requirements for a standardwafer—yet be sufficiently thin to break along the outer perimeter of thedie 20 during pick and place operations. As shown in FIGS. 4 and 5, in apreferred embodiment, the portion of the tethers 78, 178 extendingacross the open groove 76 has its smallest width adjacent to the outerperimeter 74 of the die 20. This permits the break point to be right atthe outer perimeter 74 to limit any overhang of the tether that mayresult after die separation.

[0040] Although FIGS. 4 and 5 show substantially triangular tethers, thetethers may also be of other exotic geometric shapes such assubstantially rectangular, elliptical, semi-circular, or square.Additionally, to provide better break points above trench 76, thegrooves 184 in the tethers 178 may be replaced with holes or slots inthe tethers 78 along the trench 76. Depending on the geometric shape ofthe tether, the addition of grooves, holes or slots may enable thetethers to have a better cohesive failure point along the outerperimeter 74 of the semiconductor die 20.

[0041] A process for making or forming the tethers 78, 178 for a thindie 20 on a wafer 70 will now be explained. Referring now to FIG. 6A,after forming the circuit on the die 20 on the topside 82 of the wafer70, the process includes the step of forming a cavity 88 on a backside90 of the wafer 70 (beneath the circuit on the die 20). This backsidecavity 88 defines a thin layer 92 that includes the circuit on the die20. The backside cavity 88 will also define the wafer support body 72that is substantially thicker than the thin layer 92 and the die 20. Thethin layer 92 has a thickness slightly greater than the die 20.

[0042] The cavity 88 on the backside 90 of the wafer 70 may be formedusing known semiconductor etching methods. In one embodiment, the cavity88 is formed using an anisotropic wet etch using chemical solutions suchas KOH, EDP or TMAH. A masking material (not shown) such as silicondioxide or silicon nitride may be used for etching the cavity 88. Thedepth of the cavity 88 on the backside 90 of the wafer 70 is applicationspecific and will depend on the desired thickness of the die 20. In oneexample, where the desired thickness of the die 20 is to be about 15 μm,etching may be performed for sufficient time to define the thin layer 92to a thickness of about 22 μm.

[0043] As shown in FIG. 6B, the next step is the formation of a trench76 around the circuit on the topside 82 of the wafer 70. As mentionedabove, the trench 76 will define the outer perimeter 74 of the die 20having a circuit. The trench 76 may be formed using known semiconductoretching methods. In one embodiment, the trench 76 is formed using anetch process such as reactive ion etching (RIE), plasma etching orsputter etching. The depth of the trench 76 is application specific andwill depend on the desired thickness of the die 20 and the thickness ofthe thin layer 92. The trench 76 should have a depth of at least thedesired thickness of the die 20 but smaller than the thickness of thethin layer 92 illustrated in FIG. 6A. In the above example where thedesired thickness of the die 20 is to be about 15 μm and the thin layer92 is about 22 μm, the trench 76 may be formed to about 18 μm deep.

[0044] As shown in FIG. 6C, the process also includes a step of formingtethers 78 on the topside 82 of the wafer 70. The tethers 78 also extendacross and into select portions of the trench 76 and between the wafersupport body 72 and the die 20. The tethers 78 should be patterned. Asdescribed above, FIGS. 4 and 5 show top views of some embodiments ofpatterned tethers 78, 178. Note that FIG. 6C uses the reference numberfor the tethers 78 in FIG. 4. However, the view shown in FIG. 6C wouldapply equally to the tethers 178 shown in FIG. 5 and even for othergeometric shapes of tethers. What is critical is that the tethers form abridge or other connection between the support body 72 and the thin die20 of the wafer 70.

[0045] In one embodiment, the tethers 78 are made of a polyimidematerial although other materials may be used such as otherthermoplastic materials or polymers. A polyimide material is preferredbecause it can have a thickness ranging from a few microns to tens ofmicrons. Although the thickness of the tether may be applicationspecific, in one embodiment, a polyimide tether may be between 2-10 μmon the topside 82 of the wafer 70 and 5-30 μm in the trench 76. Thepolyimide coating is preferably applied to the wafer 70 using a spincoating process. Although a spin coating process provides gooduniformity and coating qualities, other known application techniquescould be used such as spray, drop coating, and roller.

[0046] To perform the patterning, a photosensitive polyimide may beused. Existing photosensitive polyimides permit the patterning ofrelatively fine features. The patterning process may include spincoating the polyimide and a drying step by hot plates or an oven. Incombination with a negative tone photo mask, the depositedphotosensitive polyimide layer may then be exposed to a standard I or Glithography tool. The patterned polyimide tethers may then be cured byconventional methods. Curing the polyimide film involves the removal ofthe solvent carrier or other volatiles from the polyimide layer and thehardening of the polymer into suitable tethers.

[0047] If a photosensitive polyimide is not used, other methods ofpatterning may be used such as conventional wet or dry etchingprocesses. A wet etching process will typically include that thepolyimide be patterned prior to final cure. A dry etching processing mayalso include that the polyimide be patterned prior to final cure.

[0048] As shown in FIG. 6D, the next step is to remove a thin layer 94on the backside 90 of the wafer 70. The removed thin layer 94 is shownin FIG. 6D as a dashed line below the thin die 20 and along the widenedcavity 88. The removal of the thin layer 94 exposes the trench 76 butleaves the tethers 78 intact. In effect, the removal of the thin layer94 removes a portion of the thin layer 92 shown in FIG. 6A to define abottom surface 75 of the die 20. The thin layer 94 may be removed usinga variety of conventional etching methods such as reactive ion etching(RIE), plasma etching or sputter etching. The depth of the removed thinlayer 94 is application specific and will depend on the desiredthickness of the die 20, the thickness of the initial thin layer 92, andthe depth of the trench 76. As explained above, the removed thin layer94 should have a depth sufficient to expose the trench 76 but not asdeep to remove the tethers 78. In the above example where the desiredthickness of the die 20 is to be about 15 μm, the initial thin layer 92being about 22 μm, and the trench 76 being about 18 μm, the removed thinlayer 94 has a depth of about 7 μm deep.

[0049] It can be seen in the figures that the thin semiconductor die 20is still attached to the surrounding support body 72 of the wafer 70 bythe tethers 78. The wafer 70 (having at least one die 20 and tethers 78)are now suitable for packing, shipping and transporting to assemblyplants where the thin dice may be subsequent separated by breaking thetethers 78 by pick and place operations. These further operations areexplained in more detail below. What has been described is a device andprocess that helps in subsequent wafer separation of thin dice. Thestructure of the wafer also makes it easier to ship and automate diepick and place operations. This process also allows the surfaces of thedie to be maintained very clean prior to die attachment to othersurfaces.

[0050] The above figures illustrate a thin die that is substantiallysquare. It is noted that the present invention is not limited to thindice that are substantially square. While a square die is illustrated,in some circumstances it may be desirable to use other geometricalshapes for the die. Moreover, although the procedures are described inthe context of a silicon-based semiconductor material, the presentinvention may also apply to the formation of tethers on other types ofsemiconductor materials such as gallium arsenide (GaAs). One of ordinaryskill in the art with the benefit of this application would realize thatsuch other geometrical shapes and semiconductor materials could be used.

[0051] Separation and Handling of a Thin Die

[0052] As described above, the thin die 20 is suspended on the wafer 70by thin tethers 78, 178 that can be made of a material such aspolyimide. As will be illustrated below, the tethers allow a cohesivefailure point that occurs along the outer perimeter 74 of the thin die20 during subsequent pick and place operations. It is preferred that theindividual tethers 78, 178 be small to minimize the amount of residualpolyimide left on the area extending on the die 20. The number oftethers 78, 178 around the perimeter of the thin die 20 should besufficient to ensure that the die 20 does not fall off during waferhandling and shipping. As described above, in one embodiment, the thindie 20 is attached to the wafer by four (4) tethers 78, 178, one tether78, 178 for each side of the die 20.

[0053] An advantage of suspending a thin die 20 by tethers 78, 178 isthat the die 20 is ready for pick and place operations without anyfurther processing steps at the wafer level. Additionally, suspendingthe thin die 20 by use of tethers 78, 178 enables the backside of thedie 20 to be more easily shielded from contaminants. As will beexplained in more detail below, in one embodiment for a pressure sensor,the backside of the die 20 is the portion of the die 20 that is bondedor otherwise attached to a pressure port. This backside surface needs tobe clean from contaminants for sensors.

[0054] A new process for separating and handling thin die is illustratedin FIGS. 9A-9D and 10A-10D. FIGS. 9A-9D illustrate a process to removeor otherwise separate a thin die 20 from a wafer 70. FIGS. 10A-10Billustrate a process of transporting the thin die 20 from the wafer 70and placing the die 20 on a surface or diaphragm 40. As illustrated inFIGS. 7 and 8, some of the tools used to perform these processes are abacking 110 and a die handler 120.

[0055]FIG. 7 shows one embodiment of a suitable backing 110. The backing110 is preferably made of a metallic or other rigid material such asaluminum. During general wafer handling and die removal, the backing 110is rigidly clamped, taped or otherwise attached to the wafer 70. Thebacking 110 has an array or plurality of holes 112. The holes 112 arespaced out to line up exactly with the plurality of backside cavities 88that are formed on the wafer 70 described above in FIG. 6A. The contoursof the backing 110 are shaped like standard wafer frames. This allowsthe backing 110 to fit inside standard feeders and machine fixtures. Theuse of a rigid backing 110 is important because the wafer 70 itself hasvery little flexural strength due to the numerous dice surrounded bysquare holes or trenches 76. The trenches 76 leave behind a thin waferskeleton that may be subject to fracture without the use of the backing110. Additionally, the trenches 76 have sharp corners that act as stressconcentrators. The purpose of the backing 110 is to protect the wafer 70against fracture during transporting and handling as well as during dieremoval. The holes 112 in the backing 110 allow an ejection pin (asshown in FIGS. 9A-9D) to move freely in and out of every backside cavity88 of the wafer 70.

[0056]FIG. 8 shows one embodiment of a die handler 120. The die handler120 is used for pick and place operations. Where the die 20 is used fora pressure sensor, the die handler 120 may also be used to remove orseparate the die 20 from a wafer 70 and install the die 20 to a pressureport or diaphragm 40. In other applications, the die handler 120 may beused to remove or separate the die from the wafer and install the die towhatever other surface that the die is to be mounted. The die handler120 may be attached to an automated machine to perform pick and placeoperations to each of the plurality of dice 20 on the wafer 70.

[0057] In one embodiment, as shown in FIG. 8, the die handler 120 mayhave an upper body chamber 122, a rigid body portion 124, a movable bodyportion 126, and a tip 128. The upper body chamber 122 is attached tothe rigid body portion 124. As shown in FIG. 8, this may be done bythreading the upper body chamber 122 to the rigid body portion 124. Theupper body chamber 122 has a port 130 that is configured for receiving aline to a vacuum source (not shown). The upper body chamber 122 ispreferably made of plastic but may be made of other materials such asmetallic materials.

[0058] The movable body portion 126 is movably attached to the rigidbody portion 124. In one embodiment, the movable body portion 126 hasguide pins 132 that are capable of sliding within cylindrical chambers134 of the rigid body portion 124. The movable body portion 126 iscapable of moving up and down in relation to the rigid body portion 124.Between the movable body portion 126 and the rigid body portion 124 is apiston 136. The piston 136 is rigidly attached to the movable bodyportion 126 and movably attached to the rigid body portion 124 withincylindrical chambers 138 and 140. The piston 136 has a ridge 142 thatallows the piston to be retained in the rigid body portion 124. A spring144 is used within a cylindrical chamber 140 in the rigid body portion124 to provide a compressive force to keep the piston 136 and movablebody portion 126 in the downward position. The rigid body portion 124,the movable body portion 126, and the piston 136 are preferably made ofa metallic material such as aluminum, although other materials may beused such as plastic.

[0059] The tip 128 is preferably made of a flexible material such asrubber. The tip 128 is attached to the movably body portion 126. Asdescribed and shown in FIG. 8, the spring 144 is configured to allow thetip 128 of the handler 120 to also move in relation to the rigid bodyportion 124.

[0060] As explained above, the port 130 in the upper body chamber 122 isconfigured for receiving a line to a vacuum source. A passageway 146 isprovided through the rigid body portion 124, through the piston 136,through the movable body portion 126, and through the tip 128. As willbe explained below, this passageway 146 provides a vacuum suction forcethat will assist in pick and place operations for the thin die 20.

[0061] Other configurations for a die handler 120 may be suitable forthe present invention. For example, the guide pins 132 may be rigidlyattached to the rigid body portion 124 and extend into cylindricalchambers in the movable body portion 126. Alternatively, the upper bodychamber 122 may be removed and the port 130 (attached to the vacuumsource) may be directly connected to the passageway 146 of the rigidbody portion 124. In any event, what is important is that the diehandler 120 has some flexibility when pressure is applied to the thindie 20 during pick and place operations. Some of those features mayinclude fabricating the tip 128 out of a flexible material such asrubber. Alternatively, the die handler 120 could include a springmechanism such as that described in relation to FIG. 8.

[0062] What will now be explained is a procedure for pick and placeoperations for separating and handling a thin die 20. Again, forpurposes of illustration, the description and figures are shown in thecontext of the thin semiconductor die 20 described above in FIG. 1. Oneof ordinary skill in the art with the benefit of this disclosure willrecognize, however, that the present invention applies to other thin dieapplications.

[0063] Referring to FIG. 9A-9D, a procedure for removing or separatingthe thin die 20 from the support body 72 of the wafer 70 will beexplained. The thin die 20 is initially attached to the support body 72by an attachment mechanism such as that described above having aplurality of tethers 78, 178. As illustrated in FIG. 9A, the wafer 70(having a support body 72, at least one thin die 20, and tethers 78 (or178)) is positioned on backing 110. One of the plurality of holes 112 ispositioned beneath the thin die 20. The tip 128 of the die handler 120is positioned above the thin die 20 on the wafer 70. Because no force isbeing exerted on the tip 128, the spring 144 within chamber 140 keepsthe tip 128 in the downward position by forcing the ridge 142 of thepiston 136 to the bottom of the cylindrical chamber 140 of the rigidbody portion 124. The vacuum source connected to the port 130 is thenactivated providing a vacuum to passageway 146. An ejection pin 150 isalso positioned in a spaced apart relationship beneath the thin die 20and within a hole 112 of the backing 110.

[0064] Referring to FIG. 9B, the die handler 120 is then moved in thedownward direction (as shown by arrow A) toward the thin die 20. The tip128 of the die handler 120 makes contact with the thin die 20. The tip128 of the die handler 120 continues in the downward direction A tobreak the tethers 78 (or 178). The rigid backing 110 holds the supportbody 72 of the wafer 70 in place. This separates the thin die 20 fromthe support body 72 of the wafer 70. The tip 128 of the die handler 120continues in the downward direction A until it makes contact with theejection pin 150. This clamps the thin die 20 between the tip 128 of thehandler 120 and the ejection pin 150. It is noted that when the tip 128of the die handler 120 travels in the downward direction A (to break thetethers and make contact with the ejection pin 150), the piston 136 ispermitted to move within the chamber 140 in an upward direction (asshown by arrow B) to compress the spring 144. This provides a softlanding of the tip 128 when it comes in contact with the thin die 20 toprevent damage.

[0065] It is noted that the thin die 20 is detached from the wafer 70 byexerting a downward pressure. The application of a downward force is animportant feature of the present invention. An alternative process suchas pulling the thin die 20 up by relying solely on the suction forceexerted by the passageway 146 within the tip 128 has proven to beunreliable. This is due to the fact that a very small contact area ofthe die 20 limits the suction force that can be exerted by thepassageway 146 within the tip 128. Relying solely on the suction forceto detach the die 20 would limit the tether design to being extremelyweak. This would result in requiring tight process controls on tethermanufacturing and would increase the risk of die separation during waferhandling and shipping. In contrast, relying on a compressive force(against the rigid backing 110) to break off the tether allows moreflexibility in varying the tether design. It also allows more tolerancein variability in the tether fabrication process without compromisingthe ability to separate the die 20 from the wafer 70.

[0066] In the preferred embodiment, the vacuum source remains activethrough passageway 146 while the thin die 20 is detached from the wafer70. If the vacuum is turned off, the thin die 20 may not be heldhorizontally during the breakage of the tethers. This may cause thetethers to break at different times. If the tethers do not breaksimultaneously, there is a risk that the last tether will fold and actas a hinge, leaving the thin die 20 hanging by one edge.

[0067] Referring to FIG. 9C, the tip 128 of the die handler 120 andejection pin 150 move together in the upward direction (as shown byarrow C). This may further move the piston 136 within the chamber 140 inan upward direction (as shown by arrow D) to further compress the spring144. It is noted that during this step the thin die 20 is preferablyclamped between the tip 128 of the die handler 120 and the ejection pin150. The clamped thin die 20 is then extracted from the support body 72of the wafer 70 by a simultaneous upward motion in the upward directionC. Left unclamped, the thin die 20 may be lost during extraction, as thedie 20 may come in contact with the residual tethers 78 (or 178) lefthanging around the perimeter of the support body 72 of the wafer 70.Again, this is due to the fact that the net suction force exerted by thevacuum source through the passageway 146 within the tip 128 may not bestrong enough to pull the thin chip 20 through any residual tethers lefthanging around the perimeter of the support body 72 of the wafer 70.

[0068] Clamping the thin die 20 between the tip 128 and the ejection pin150 also eliminates the possibility that the die 20 will shift or rotatebefore or during extraction. Such shifts or rotations could possiblycause the thin die 20 to collide with the support body 72 of the wafer70. Additionally, to minimize bending or shearing, the ejection pinshould have a diameter in close proximity to that of the tip 128 and itsupper surface should be flat in relation to the thin die 20.

[0069] The die handler 120 and the ejection pin 150 may move together inan upward direction C to extract the thin die 20. Alternatively, havinga spring 144 in the die handler 120, the die handler 120 could beprogrammed to be stationary while the ejection pin 150 provides theupward force. The spring 144 enables the ejection pin 150 to provide theupward force by allowing the tip 128 of the die handler 120 to moveupward with the ejection pin 150.

[0070] Referring to FIG. 9D, the die handler 120 may be moved in theupward direction to lift the thin die 20 off the ejection pin 150. Thiswill move the piston 136 within the chamber 140 of the die handler 120in a downward direction (as shown by arrow E) by the compressive forcesexerted by the spring 144. The ridge 142 of the piston 136 will thenrest in the bottom of the chamber 140. With the vacuum source to thepassageway 146 still active, the thin die 20 remains on the tip 128 ofthe die handler 120. The ejection pin 150 is now free to retract in itinitial downward position.

[0071] As can be seen in the above-described separation and extractionprocess, the use of a spring-mounted compliant pick up head has severalimportant advantages. First, the soft spring limits the force when thetip 128 makes initial contact with the thin die 20. Second, the softspring limits the clamping force exerted on the thin die 20 when thethin die 20 is clamped between the tip 128 and the ejection pin 150.This reduces the risk of damage to the thin die 20. Third, the springeliminates the need to synchronize the upward motions of the ejectionpin 150 and the tip 128 of the die handler 120 as shown in FIG. 9C. Ifthe tip 128 were rigid, synchronizing these two moving parts whilecontrolling the clamping force would be difficult to achieve. Now, thetip 128 of the die handler 120 can be programmed to be stationary whilethe ejection pin 150 moves upward. Forth, the soft spring allows the diehandler 120 to be operated in displacement control, without any need tomonitor the clamping force. Finally, the soft spring loosens therequirements on the precision and accuracy of the stopping positions ofboth the tip 128 of the die handler 120 and the ejection pin 150.

[0072] What will now be explained is a procedure for handling andinstalling a thin die 20 on a diaphragm 40. As illustrated in FIGS. 10Aand 10B, the thin die 20 (attached to the tip 128 of the die handler120) is moved from the support body 72 of the wafer 70 and positionedabove a diaphragm 40. It is noted that during this handling procedure,the vacuum source attached to the passageway 146 within the tip 128 isactive. As shown in FIG. 10C, the tip 128 of the die handler 120 ismoved in the downward direction (as shown by arrow F) to place the thindie 20 to the diaphragm 40 via a coupling 50 (explained below). It isnoted that when the thin die 20 and the tip 128 of the die handler 120travels in the downward direction F (and make contact with the diaphragm40 and coupling 50), the piston 136 is permitted to move within thechamber 140 in an upward direction (as shown by arrow G) to compress thespring 144. This provides a soft landing of the thin die 20 when itcomes in contact with the coupling 50 and the diaphragm 40 to preventdamage.

[0073] As explained above, in the case of a thin die 20 for a pressuresensor, a pressure-conductive coupling 50 is used between the thin die20 and the diaphragm 40. The coupling 50 may include acorrosive-resistant pressure-conductive electrically insulating materialto conduct and transmit the sensed pressure from the diaphragm to thethin semiconductor-sensing die 20. A suitable coupling 50 is made offused glass frit and screen-printed on the diaphragm 40. Glass flit isuseful because it electrically isolates and prevents shorts from themetal diaphragm 40.

[0074] Referring to FIG. 10D, after the thin die 20 is placed on thediaphragm 40 via coupling 50, the die handler 120 is moved in the upwarddirection (as shown by arrow H). Prior to moving in the upward directionH, the vacuum source to the passageway 146 within the tip 128 is turnedor switched off. This allows the tip 128 of the die handler 120 toseparate from the thin die 20. The die handler 120 is now ready toperform its next pick and place operation on a new die on the wafer 70.

[0075] Although FIGS. 10A-10D show the handling and installing of a thindie 20 on a diaphragm 40, one of ordinary skill in the art having thebenefit of this disclosure would realize that the same handling andinstalling steps may be taken to mount a thin die on other surfaces forother applications.

[0076] What has been described is a new device and process forseparating and handling a thin die on a wafer. The present inventionpermits the separation of a thin die handled and shipped on the originalwafer. The thin die can be separated or extracted directly from theoriginal wafer used to form the integrated circuit on the die.Additional steps at the wafer level are avoided before the pick andplace operations.

[0077] The above description of the present invention is intended to beexemplary only and is not intended to limit the scope of any patentissuing from this application. For example, the present discussion useda thin die for a sensor to describe the separation and handling of athin die. The present invention is also applicable to separation andhandling of other types of thin die such as applications for smartcards, smart labels, actuators, and multi-thin wafer designs. Thepresent invention is intended to be limited only by the scope and spiritof the following claims.

What is claimed is:
 1. A semiconductor wafer comprising: a support bodymade of a semiconductor material; at least one thin die having a circuitformed thereon, the thin die having an outer perimeter defined by anopen trench, the open trench separating the thin die from the supportbody; and a plurality of tethers extending across the open trench andbetween the support body and the at least one thin die.
 2. Thesemiconductor wafer of claim 1 wherein the support body has a firstthickness and the at least one thin die has a second thickness, thesecond thickness being substantially less than the first thickness. 3.The semiconductor wafer of claim 1 wherein at least one of the pluralityof tethers is substantially triangular in shape.
 4. The semiconductorwafer of claim 3 wherein the at least one substantially triangulartether has a base and a tip, the base of the tether being attached tothe support body of the wafer and the tip of the tether extending acrossthe trench and attached to the at least one thin die.
 5. Thesemiconductor wafer of claim 1 wherein at least one of the plurality oftethers has a portion that extends across the open trench, the portionextending across the open trench having its smallest width adjacent tothe outer perimeter of the at least one thin die.
 6. The semiconductorwafer of claim 1 wherein at least one of the plurality of tethers has aportion that extends across the open trench, the portion extendingacross the open trench having at least a portion of a groove.
 7. Thesemiconductor wafer of claim 1 wherein at least one of the plurality oftethers has a portion that extends across the open trench, the portionextending across the open trench having at least a portion of a hole. 8.The semiconductor wafer of claim 1 wherein the circuit of the die isadapted for a pressure sensor.
 9. The semiconductor wafer of claim 1wherein the plurality of tethers are made of a polyimide material.
 10. Awafer comprising: a support body made of a semiconductor material; atleast one thin semiconductor die having a circuit formed thereon, thethin semiconductor die having an outer perimeter defined by an opentrench, the open trench separating the thin semiconductor die from thesupport body; and a means for attaching the outer perimeter of the atleast one thin semiconductor die to the support body across the opentrench.
 11. The wafer of claim 10 wherein the means for attaching theouter perimeter of the at least one thin semiconductor die to thesupport body across the open trench includes a plurality of tethers. 12.The wafer of claim 11 wherein the tethers are made of a polyimidematerial.
 13. A method of making a thin die on a wafer, the wafer havinga support body, a topside and a backside, a circuit formed on thetopside of the wafer, the method comprising the steps of: forming acavity on the backside of the wafer beneath the circuit that defines afirst layer, the first layer includes the circuit; forming a trencharound the circuit on the topside of the wafer that defines an outerperimeter of the thin die; forming a plurality of tethers that extendacross the trench and between the wafer support body and the thin die;and removing a portion of the first layer to define the bottom surfaceof the thin die.
 14. The method of claim 13 wherein the step of formingthe cavity on the backside of the wafer includes wet etching thebackside of the wafer.
 15. The method of claim 13 wherein the step offorming the trench on the topside of the wafer includes reactive ionetching to form the trench.
 16. The method of claim 13 wherein thetethers are made of a polyimide material.
 17. The method of claim 13wherein the step of forming of the tethers includes patterning thetethers so they are substantially triangular.
 18. The method of claim 13wherein the step of removing the portion of the first layer includesreactive ion etching the first layer.
 19. A method of forming tethers ona wafer to retain a thin die to a support body of the wafer, the waferhaving a topside and a backside, the thin die positioned adjacent to thetopside of the wafer, the method comprising the steps of: forming acavity on the backside of the wafer beneath the thin die that defines afirst layer, the first layer includes the thin die; forming a trencharound the thin die on the topside of the wafer that defines an outerperimeter of the thin die and extends between the thin die and thesupport body; patterning a polyimide material on the top surface of thewafer to define the tethers, the tethers extending across the trench andbetween the thin die and the support body; and removing a portion of thefirst layer to expose the trench such that the tethers provide theattachment between the thin die and the support body.
 20. The method ofclaim 19 wherein the step of patterning a polyimide material on the topsurface of the wafer defines the tethers in a substantially triangularshape.